When hydrogen is used as a fuel in motor vehicles, a hydrogen fuel depot infrastructure for refueling must also be developed. Typically, in the use of hydrogen to fuel motor vehicles powered by fuel cells, or in the use of compressed natural gas (CNG) to power internal combustion engines in motor vehicles, present practice is that hydrogen is stored in refillable on board fuel tanks having a maximum design pressure in the range of about 5000 psi, CNG is stored in tanks having a maximum design pressure in the range of about 3600 psi. Pressures exceeding 3600 psi and 5000 psi and in the range of about 10,000 psi or more are likely to be utilized as the use of high pressure gas technology becomes more widespread. An increase in the energy efficiency of the overall system of fuel depots, vehicle tanks, refueling systems, and their interrelationships is a desirable goal.
Hydrogen powered vehicles utilize light weight polymer/composite hydrogen storage tanks to store hydrogen fuel on board at high pressure. Herein, reference to hydrogen powered vehicles correlates with the use of the invention with compressed natural gas powered vehicles (CNGVs). For clarity and convenience, hydrogen is referred to in the specification and is a term intended to be interchangeable, generally evident in context, with compressed natural gas, high pressure gas, or gas. The use of multiple cylindrically shaped small tanks rather than one large tank is preferred for vehicle design purposes. Various designs for high pressure hydrogen refueling systems have been proposed. When a hydrogen powered vehicle is filled with hydrogen, the pressurized on board storage tanks may be characterized as including chemical energy from the hydrogen itself (consumed in powering the vehicle), and mechanical and thermal energy associated with the high pressure under which the hydrogen gas enters the tank at the refuel depot and the high pressure under which the gas is refueled and stored the vehicle tank[s].
During a high pressure refueling process with hydrogen or CNG, the interior of the on board tanks, namely, the gas itself, becomes heated as a result of gas compression as the tank pressure increases and other refueling parameters affect the process. [After refueling, the tank interior gas temperature and pressure decrease slowly as the fuel gas is consumed during vehicle operation.] Conventionally, it is not usually possible to obtain a full refill tank pressure to a high pressure design maximum without pressure compensation during the course of refueling, namely, the charge of fuel input into and stored in the tank must be initially in excess of the optimum design tank pressure because of the compression/heating effect caused by the high pressure compression of gas in the tank as a result of refueling. As a result, vehicle mileage in terms of vehicle range is reduced; the use of higher design pressures worsens this condition. Solutions have been proposed to resolve this under fill problem. A slower flow rate during refill results in a lower initial tank temperature, however, a slow fill, is undesirable, and may be impractical when significant numbers of users are involved. An undesirable consequence of a slower flow rate during refueling to avoid heat build up is a longer refueling time. Another option is to cool the gas before refueling; however, pre-cooling requires substantial energy, thereby reducing the overall energy efficiency of a hydrogen economy. A pressure overfill as another option requires an additional energy expense in gas compression, exacerbating the heat generated in the tank as a result of high pressure compression. In any case, secondary treatment of the gas is generally unnecessary when fill pressures are at 5000 psi or lower. As tank pressures exceed 3600 psi and 5000 psi and approach or exceed 10,000 psi, cooling becomes an important factor in the refueling process to achieve a full tank capacity fill. When a full fill is achieved, overall vehicle range per each tank refill thereby increases, energy required for a refill (such as for precooling or a pressure overfill) is reduced, and overall customer satisfaction increases.